An image forming apparatus which forms a composite image of a monochrome image with additional information of a particular pattern has been known. The particular pattern referred to herein means a color pattern superimposed on a monochrome copy image of a monochrome original read by a reading scanner, and more precisely, color companies logs and texts exhibiting the nature of documents, such as “CONFIDENTIAL” and “URGENT”.
Another type of image forming apparatus which forms a composite image of a color image with additional information of a pattern image (trace pattern), that is, identification information identifying a particular device, has been also known. This type of image forming apparatus was devised to prevent forgery of particular kinds of images including securities, paper money, etc. by an abuse of today's sophisticated image forming apparatus capable of forming high-quality color images, such as a color copying machine and a color printer. The trace pattern is superimposed on a copy image in an almost unnoticeable color to human eyes, for example, yellow.
Techniques related to the foregoing color image forming apparatuses are disclosed in, for example, Japanese Patent No. 2614369 effected on May 28, 1997 (U.S. Pat. No. 5,257,119 effected on Oct. 26, 1993, U.S. Pat. No. 5,465,161 effected on Nov. 7, 1995, U.S. Pat. No. 5,737,100 effected on Apr. 7, 1998, and U.S. Pat. No. 5,472,408 effected on Apr. 21, 1998) etc.
On the other hand, a color image forming apparatus, such as a color copying machine and a color printer, is chiefly divided into two types: one adopts a multi-rotation process method and the other adopts a tandem method.
A color image forming apparatus adopting the multi-rotation process method includes one photosensitive body, and four developing devices withholding yellow, magenta, cyan and black toners are driven sequentially in association with the rotation of the photosensitive body. Therefore, by this method, a color image is formed by repeating an image forming process four times in total.
In case of forming a monochrome image, the multi-rotation process method is advantageous over the tandem method in that adaptation to compact apparatuses, a shorter printing time, and improved throughput (the number of copies per unit time). On the other hand, in case of forming a color image, the multi-rotation process method has a problem that not only the print time is extended, but also the throughput is reduced.
Meanwhile, a color image forming apparatus adopting the tandem method includes four image forming stations, each comprising a photosensitive body and a developing device withholding toner of one of four colors: yellow, magenta, cyan and black. Therefore, by this method, a color image is formed by driving each image forming station separately.
In case of forming a full-color image, the tandem method can attain high throughput compared with the multi-rotation process method. However, in case of forming a monochrome image, the throughput is reduced compared with the multi-rotation process method because a paper has to travel a longer transportation path. As to the tandem method, an arrangement to increase a processing rate and hence a printing rate only when forming a monochrome image has been proposed.
The tandem method has another problem that, because image forming process devices, such as a charger and a laser scanner unit, have to be provided to each image forming station, the size and costs of the apparatus as a whole are increased undesirably.
Further, a compact and inexpensive color image forming apparatus including two photosensitive bodies has been developed. This type of color image forming apparatus can attain higher throughput than the one adopting the multi-rotation process method when making color image copies, and also can attain higher throughput than the one adopting the tandem method when making monochrome copy images.
As to the foregoing color image forming apparatus including two photosensitive bodies (hereinafter, referred to as double-photosensitive-body method), techniques disclosed in the following publications (a) and (b) have been known.
(a) Japanese Laid-open Patent Application No. 12273/1990 (Japanese Official Gazette, Tokukaihei No. 2-12273, publishing date: Jan. 17, 1990)
According to the technique disclosed in this publication, an image of a first color and an image of a second color are formed respectively on two photosensitive bodies, and transferred sequentially onto a paper attracted to a transfer drum. Then, an image of a third color and an image of a fourth color are formed respectively on the two photosensitive bodies, and transferred sequentially onto the paper attracted to the transfer drum, whereby a color image is formed on the paper.
(b) Japanese Laid-open Patent Application No. 204871/1992 (Japanese Official Gazette, Tokukaihei No. 4-204871, publishing date: Jul. 27, 1992), and Japanese Laid-open Patent Application No. 177286/1998 (Japanese Official Gazette, Tokukaihei No. 10-177286, publishing date: Jun. 30, 1998)
According to the technique disclosed in these publications, an image of a first color and an image of a second color are formed respectively on two photosensitive bodies, and transferred sequentially onto an immediate transfer belt. Then, an image of a third color and an image of a fourth color are formed respectively on the two photosensitive bodies, and transferred sequentially onto the intermediate transfer belt. Finally, an image formed on the intermediate transfer belt is transferred onto a paper, whereby a color image is formed on the paper.
Incidentally, an image forming apparatus which forms a composite image by superimposing a particular pattern on a monochrome image can adopt any of the foregoing multi-rotation process method, tandem method, and double-photosensitive-body method.
However, combinations of the foregoing arrangement such that gives a particular pattern to a monochrome image with the foregoing adoptable methods to the image forming apparatus have not been fully discussed. Thus, some combinations may cause a new problem that the printing time required for a monochrome copy image is extended undesirably by giving a particular pattern to the monochrome copy image depending on the selected method.
Further, a particular pattern to be superimposed on a monochrome copy image of a monochrome original is not necessarily a full-color image, and there may be a case where such a particular pattern is a monocolor other than black, that is, merely a dichromatic copy image of black and one more different color has to be formed. However, such a case has not been concerned at all.
In addition, the foregoing technique of producing a composite image with the trace pattern was devised only for a color image, and the use for a monochrome image has not been discussed. However, in practice, besides the color images including securities, paper money, etc., prohibition against unauthorized duplication should be also concerned for documents of secret information. Therefore, a mode to superimpose a trace pattern on a monochrome image should be also provided while maintaining a printing rate as fast as a monochrome image printing rate.